1. Field of Invention
The invention relates generally to plasma processing of mass produced devices such as integrated circuits. The invention relates more specifically to the problem of maintaining repeatable plasma chemistries during repeated executions of a given plasma-processing recipe. The invention relates yet more specifically to high-density plasma (HDP) processes for oxide etching wherein so-called consumable scavenger-emitting kit parts are employed.
2. Cross Reference to Related Applications
The following U.S. patent applications are assigned to the assignee of the present application, are related to the present application and their disclosures are incorporated herein by reference:
(A) Ser. No. 08/138,060 Attorney Docket No. AM603! filed Oct. 15, 1993 by M. Rice et al and entitled, PLASMA ETCH APPARATUS WITH HEATED SCAVENGING SURFACES;
(B) Ser. No. 07/984,045 EPO Pub. No. 0601468A1, Attorney Docket No. AM400! filed Dec. 1, 1992 by K. Collins et al and entitled, PROCESS AND ELECTROMAGNETICALLY COUPLED PLANAR PLASMA APPARATUS FOR ETCHING OXIDES;
(C) Ser. No. 07/941,507 EPO Pub. No. 0552491A1, Attorney Docket No. AM306-2! filed Sep. 8, 1992 by K. S. Collins et al. and entitled, PLASMA ETCH PROCESS;
(D) Ser. No. 07/824,856 filed Jan. 24, 1992 by K. S. Collins et al; and
(E) Ser. No. 07/722,340 Attorney Docket No. AM306! filed Jun. 27, 1991 by K. S. Collins et al.
3. Description of Related Art
In the mass production of semiconductor integrated circuits (IC's) and the like (e.g., integrated optical devices, micromachines, etc.) it is generally desirable to maintain precisely repeatable results from one wafer or other workpiece to the next.
To achieve such repeatability it is generally desirable to maintain substantially similar proportionalities of chemical mixtures and substantially similar reaction rates within chemical process steps from one wafer or other workpiece to the next.
One process that calls for precise repeatability of chemistries and rates is a plasma etching step wherein oxide layers such as those composed of silicon dioxide and found on silicon-based integrated circuit wafers are selectively etched by a chemically reactive plasma.
In one such form of oxide etching, a high-density plasma (HDP) is used for carrying out reactive ion etching (RIE) of mask-exposed surfaces.
HDP etching is characterized by a relatively-expansive plasma cloud that extends throughout the reaction chamber rather than being constrained to the vicinity of a workpiece surface.
The ubiquitous nature of the HDP cloud is sometimes exploited to provide enhanced chemical selectivity through the use of so-called `hot consumable plasma-interactive parts`. In fluorine-based oxide etching for example, the plasma-interactive parts are sometimes referred to as `scavenger-emitting parts`. Such scavenger-emitting parts interact with an impinging plasma and alter the chemical balance of the plasma in favor of etching away certain targeted oxide materials as opposed to undesired etching away of other materials.
The underlying technology behind scavenger-emitting parts is detailed in the above-cited U.S. application Ser. No. 08/138,060.
In brief, silicon-containing `scavenger-emitting parts` composed of quartz (silicon dioxide) or other silicon-containing materials (e.g., monocrystalline silicon) are installed within and lining the interior of the plasma reaction chamber. When heated by an impinging plasma, the scavenger-emitting parts emit molecularly-unbound silicon particles from their surfaces at temperature-related rates.
The heat-released silicon particles enter the plasma cloud as `scavengers` and chemically combine with some of the halogen radicals, such as free fluorine radicals (e.g., F.sup.-, F.sup.0), that happen to be present within the cloud at the time.
This chemical capture mechanism works to advantageously reduce the number of free halogen radicals present within the plasma etch cloud and to thereby shift the proportionalities of chemicals found within the plasma cloud in favor of non-halogens.
Although it is often desirable to have a limited number of free-halogen radicals (e.g., F.sup.-) within an etch plasma, high concentrations of such free-halogen radicals tend to attack undiscriminatingly. They etch away nontargetted materials (e.g., oxygenless layers such as those composed of mono or polycrystalline silicon) instead of selectively etching away only the targeted materials (e.g., oxygen-containing dielectrics such as silicon dioxide). Hence, excessive amounts of free-halogen radicals are undesirable.
If the free-halogen radicals of an etch plasma are mixed in appropriate proportions with other reactive agents such as the fluorocarbon radicals, CF.sub.3.sup.+ or CF.sub.3.sup.0 for example, the resulting plasma becomes more selective for etching away silicon dioxide and other oxygen containing materials as opposed to oxygenless materials.
In addition to the `scavenger-emitting` types of plasma-interactive parts, there are other alternate or supplementary types of `hot parts` that may be used for interacting with the plasma and altering mixture proportions within the plasma.
An example of an alternate such part would be a block of organic material such as Teflon.TM. that is placed in the plasma chamber to emit carbon particles into the plasma cloud when attacked by fluorine. The released carbon works to increase the amount of CF.sub.3 in the plasma cloud relative to free F radicals.
The scavenger-emitting and other types of plasma-altering hot parts are generically referred to herein as `plasma-interactive parts`.
As explained initially, it is desirable to maintain highly repeatable results from one wafer to the next in the mass production of semiconductor-based integrated circuits (IC's) and the like.
It has been found that the emission rate of each plasma-interactive part (e.g., each scavenger-emitting part) within a plasma chamber is sensitive to the localized temperature of that plasma-interactive part during plasma-processing. Thus as the part's temperature changes, the rate of scavenging or other interaction with the plasma changes. And consequentially, the chemical make up and selectivity of the plasma that is acting on an under-process workpiece changes.
Also, it has been found that certain condensable components of the plasma tend to condense on surfaces of the plasma-interactive parts if the part temperatures are below respective vaporization temperatures of such components. The condensates reduce the plasma-interactive surface area of the plasma-interactive parts.
By way of example, when scavenger-emitting parts in an HDP etch chamber remain below certain temperatures, carbonaceous components of the plasma tend to condense on the surfaces of the scavenger-emitting parts and thereby reduce the emitting surface area of the scavenger-emitting parts. Such carbonaceous condensation also removes desired organic constituents from the plasma and disadvantageously shifts chemical proportionalities within the plasma in favor of free halogen radicals, thereby creating a less selective etch mixture.
For some types of conveniently located, scavenger-emitting parts such as a silicon chamber roof or a helical-antenna dome (usually made of quartz) of an HDP etch chamber, it is possible to economically provide temperature control by way of circulating fluids or other temperature control means. See for example the above-cited U.S. application Ser. No. 08/138,060.
Unfortunately, the plasma chamber often includes other plasma-interactive parts that are less conveniently situated within the chamber interior and for which there has been heretofore no economically attractive way of maintaining constant temperature. The temperatures of these other plasma-interactive parts (referred to hereafter also as `floating kit parts`) are allowed to float up or down in accordance with localized heating and cooling mechanisms that develop as the related plasma is turned on and off for various lengths of time at various power levels.
In low pressure environments (e.g., 10 milliTorr or less) floating kit parts generally dissipate heat by way of blackbody radiation into the low pressure environment of the plasma chamber.
The floating kit parts each gain heat from the locally adjacent portion of the plasma while plasma-processing (e.g., etching) is in progress but they lose heat through blackbody radiation when plasma-processing ceases. The temperature of each floating kit part at each point in time is a function of local net heat gain or local net heat loss for that kit part as defined by the plasma-processing history.
Heretofore, when highly repeatable etch chemistries and etch rates were desired in scavenger-based HDP dielectric etchers, a plurality of sacrificial or `dummy` wafers were placed at the front end of each production lot. The HDP etch process was repeatedly carried out on the dummy wafers until steady state temperatures were obtained inside the reaction chamber for floating kit parts. Then the actual production wafers were processed under the pre-established steady state conditions.
Such resort to dummy wafers is disadvantageous in that a portion of the limited number of slots in each cassette full of production wafers is consumed by the dummy wafers. Time, materials, and production resources are disadvantageously consumed while the dummy wafers are used to reach the steady state levels of continuous processing.
Additionally, the up front use of dummy wafers in each lot does not assure that temperature repeatability of kit parts will be maintained for all remaining wafers of the lot. Other events during the processing of a given lot such as unexpected delays in the loading or unloading of wafers into and out of a plasma chamber may temporarily shift the temperatures of the kit parts away from normal steady state levels.
A better technique is needed for providing wafer-to-wafer repeatability with respect to the temperatures of floating kit parts.